Many conventional drug delivery devices expel a liquid drug from a drug container for delivery to a patient. These conventional drug delivery devices often fail to inform the patient as to the fill status of the drug container. As a result, the patient is often unaware of how much liquid drug has been provided to the patient, when a desired dose has been completed, and/or how much liquid drug remains in the drug container. Accordingly, there is a need for a monitoring system for use in drug delivery devices that can determine and provide the patient with the fill status of the drug container holding the liquid drug.